It is now generally recognized that drilling for oil and gas in a subterranean formation is a complex and sophisticated task. Many "arts" associated with the drilling are recognized as being of invaluable aid. For example, chemical analysis of formation water samples are valuable for numerous purposes. Among these are electric log interpretation and subsequent determination of the source of the produced water. However, there can be some question as to whether drill stem test samples are representative of actual formation water. This uncertainty is increased in cases where a high degree of mud filtrate invasion has occurred, since it is difficult to ascertain if the water is "produced formation water" or "produced mud filtrate".
A technique has been developed recently which utilizes a tracer in the mud which helps to ascertain the fraction of mud filtrate contained in the produced water.
One such technique uses sodium chromate in the mud as a tracer. While this method has had some degree of success one problem with the method occurs when chromium lignosulfonate is used in the drilling mud. The color bodies resulting from the presence of the lignosulfonate often give erroneous results in the chromate ion values determined colorimetrically.
My invention is directed to the solution of the problem created by the presence of lignosulfonate color bodies in drilling mud filtrates being subjected to determination of amount of chromate ion.